If compaction is achieved, is moisture still important? 2

[dirtsqueezer]

I have a sort of easy question. If compaction requirements are met, is it important to an engineer that the moisture also be within range of the optimum?

I have inspected projects where the contract documents read, "...fill shall be within 3% of optimum moisture content and be placed to 95% of theoretical maximum...". For say, a clean building sand, if you look at the proctor data, the maximum density can easily be achieved up to 5% over or under the optimum. Not that I recommend using clean SP for structural fill, but just how important is moisture at time of placement? It seems to me if compaction was met, and then a rainstorm came through and saturated the fill, you'd have the same situation as if it were placed out of the specified 'range'. Just curious how the engineers see it.

 

[Focht3]

It depends. If post-construction settlement is an issue , then the answer is probably - but not always - 'Yes'. If soil swell is an issue, then the answer is always 'Yes'.

It also depends a great deal on the shape of the moisture-density plot. A broad, flat curve on the moisture-density plot would indicate the moisture content may not be particularly critical while a sudden drop-off in the compacted density on either side of the optimum moisture content would indicate that the compaction moisture content may be critical. (The drawing's scale is important, of course!)

There are lots of things to consider -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[GeoPaveTraffic]

In this case I agree with Focht3. Whether the moisture of compacted fill is important is dependent on the purpose of that fill. Another example, if you building a dam, it is normal to compact the core material above optimum. This helps keep the core flexible thereby reducing the potential for piping through the core.

 

[BigH]

I agree too that it depends on the purpose of the fill. In road construction, once the compaction has been met (for most fills), it is not that important to maintain the moisture content (for purposes of specification). When we place base course at 5.3% or so, and after a week it is down to 3.5% or so, some clients want you to "add" water! A lot of people don't understand it means little to the dry density unless you have swelling/shrinking soils.